The invention concerns an equalization and a precompensation system for time division multiple access (TDMA) communications and its application to DECT (Digital Enhanced Cordless Telecommunications). The DECT system was conceived as a radio port interface to existing, and future, telecommunication networks, both fixed and mobile. Originally, the DECT system was conceived to be used within indoor environments, although its extension to outdoor environments has been recently proposed. However, the characteristics of outdoor multipath propagation are deeply different from those considered in the DECT standard. In particular, a non frequency selective channel was assumed as a basis for the original system design and therefore no equalization subsystems are used neither in the DECT base station (BS) nor in the user terminal, named mobile station (MS) in the context of DECT. In this scenario, to counteract the effects of multipath propagation, which are particularly severe outdoor, many base stations are located at a minimum distance ranging from 50 to 100 m, so reducing the delay spread and increasing the coherence bandwidth. The need for a very large number of DECT base stations to achieve a satisfactory performance in outdoor environments was evidenced early in the state of art, but the extremely large number of base stations which may be necessary to suitably cover a relatively large urban or suburban area leads to a huge initial network deployment cost that the service provider must afford. This is in contrast with the well-known concept of xe2x80x9ccell splittingxe2x80x9d, introduced with cellular systems in which the size of each elementary coverage area is tailored to the traffic demand so allowing a gradual deployment of the network, reducing the investment risk and allowing expenditures better matched to revenues. In order to reduce the number of necessary base stations it is mandatory to enlarge the area of the cell served by each BS. In this case, to offer a suitable quality of service, equalization techniques must be adopted both in the uplink (MS-to-BS) and in the downlink (BS-to-MS).
In principle, one of the two following approaches may be followed:
1. to add a suitable equalization technique to the DECT standard in order to improve the performance of a possible second generation of portable terminals; to ensure the compatibility with the present first generation terminals it is necessary for such a technique to be xe2x80x9ctransparentxe2x80x9d, i.e. to provide unaffected performance to existing terminals;
2. to add a xe2x80x9cproprietaryxe2x80x9d equalization technique not influencing the architecture of the user terminals, i.e. able to provide performance improvements to all DECT terminals, including those of the present generation already on the market.
Evidently, the approach described in the last point is most advantageous because performance is improved even for existing DECT terminals, while full compatibility is preserved with both present and future terminals designed in accordance with the DECT standard. It requires that the necessary modifications to the overall transmission system pertain to base stations only. It is the main objective of the present invention to provide an architecture for the base station based on equalization and precompensation means without influencing the structure of the DECT user terminal such as to ensure significant performance improvements both in the uplink and in the downlink transmissions.
Therefore the invention pertains to wireless communications, and more particularly it introduces a BS architecture which is able to provide equalization to DECT links, especially useful to those operating in outdoor environments. The quality levels of both the uplink and the downlink are simultaneously enhanced without putting any requirement to the architecture of the DECT user terminal, which therefore may be generic, i.e. of any vendor, only conditioned on adherence to the DECT standard requirements. Such results are achieved through some functions, to be added to those already performed to meet the DECT standard requirements, which may be implemented with one or more digital signal processing (DSP) special-purpose chips at the BS. Such functions are:
estimation of the channel impulse response (or, equivalently, the channel transfer function) from the received uplink signal;
equalization of the received uplink signal before decision;
time updating and frequency scaling of the estimated channel impulse response (or, equivalently, the channel transfer function);
precompensation of the downlink signal before transmission, through a suitable subsystem.
The present invention can also be used for other systems employing a TDMA scheme.